1. Field of Invention
This invention relates to the field of x-ray imaging.
2. Description of Related Art
X-ray radiation is widely used for medical x-ray imaging and non-destructive evaluation. X-ray radiation easily penetrates many materials and allows images to be taken based on the shadows of dense materials that absorb x-rays. X-ray imaging is used for both thick and thin tissue procedures in medical imaging radiology and fluoroscopy. Exemplary applications of x-ray imaging in non-destructive evaluation include the testing of buildings, structural members, pressure vessels, welds and airplane fuselage constructions, and the like for the presence of defects and structural integrity.
The application of x-ray imaging presents difficult technical problems. One particular problem is that the absorption of x-rays by materials at higher energies (greater than 100 keV) competes with the Compton scattering process. Compton scattering deflects x-rays through a small angle from their original trajectories. For imaging dense and/or thick materials, Compton-scattered x-rays can obscure the image formed by the absorption of direct unscattered x-rays.
FIG. 1 shows a conventional x-ray imaging system 20 configuration for imaging objects. The x-ray imaging system 20 comprises an x-ray source 22 and an image contrast grid (antiscatter grid) 24 placed between the x-ray source 22 and a detector 26. The x-ray source 22 emits x-rays 32 that impinge on an object 34 to be imaged. For example, the object 34 can be a human body. The transmitted x-rays 36 strike the surface 38 of the detector 26.
As shown in FIG. 2, the detector 26 may include a film cassette with a film 30 sandwiched between phosphors 28. As shown in FIG. 3, the detector 26 may alternatively include an electronic detector such as an a-Si detector 48 combined with a phosphor or photoconductor 28 as described in J. Rahn et al., xe2x80x9cHigh Resolution, High Fill Factor a-Si:H Sensor Arrays for Optical Imaging,xe2x80x9d Materials Research Society Proc. 557, April 1999, San Francisco, Calif.; and R.A. Street, xe2x80x9cX-ray Imaging Using Lead Iodide as a Semiconductor Detector,xe2x80x9dProc. SPIE 3659, Physics of Medical Imaging, Feb. 1999, San Diego, Calif., each incorporated herein by reference in its entirety.
As shown in FIG. 4, some of the non-normal x-rays 40 strike dense material 42 in the body, such as bone, and are absorbed by the dense material. However, other x-rays 44 are scattered and do not strike the dense material 42 and pass through the soft body tissue without being absorbed. These scattered x-rays are known as Compton-scattered x-rays.
The Compton-scattered x-rays 44 that do not strike dense material 42 in the object 34 adversely affect the formed image of the dense material. That is, the Compton-scattered x-rays 44 exit from the object 34 at positions that are laterally spaced from the positions at which they entered the object 34. Based on their exit locations, the Compton-scattered x-rays 44 would appear to have passed through the region of the object 34 where the dense material 42 is located, but without having been absorbed by the dense material 42.
As shown in FIG. 5, the image contrast grid 24 is provided in the x-ray imaging system 20 to absorb the Compton-scattered x-rays 44 that are not absorbed by dense material 42 in the object 34. The Compton-scattered x-rays 44 affect the darkness (contrast) of the image of the dense material 42 that is formed by the actual absorption of the x-rays 40 by the dense material 42. The image contrast grid 24 reduces the effects of the Compton-scattered x-rays 44 on the image formed by the absorption of direct x-rays by eliminating the Compton-scattered x-rays 44 that travel in a direction through the object 34 that does not point to the x-ray source 22. By eliminating the Compton-scattered x-rays 44, the image contrast is enhanced.
In general, image contrast grids are required for all xe2x80x9cthickxe2x80x9d tissue medical imaging procedures; i.e., procedures in which the screen is not located close (within about the thickness of the screen) to body tissue during medical imaging procedures.
Image contrast grids have been formed by laminating together foils of x-ray transparent material, such as aluminum, and x-ray absorbing material, such as lead, to form an extended sandwich structure. FIG. 6 illustrates a known sandwich structure image contrast grid 124 including aluminum foils 126 and lead foils 128 forming an alternating, parallel arrangement.
Other methods of forming image contrast grids have been described, for example, in U.S. Pat. Nos. 5,581,592 and 5,557,650, incorporated herein by reference in their entirety.
However, known image contrast grids, such as the image contrast grid 124, and the processes for forming the grids are unsatisfactory for at least several reasons. First, these processes are complicated and expensive to perform, leading to a high cost of the grids.
Second, known image contrast grids, such as the image contrast grid 124, have a relatively coarse structure that produces grid lines in the formed images. For example, to reduce this problem, the grids can be moved slightly back and forth in a direction 46 approximately perpendicular to the normal (i.e., the direction of the x-rays 36) to blur the image of the grid lines formed on the film. This movement of the grids is known as the xe2x80x9cBucky system.xe2x80x9d However, the Bucky system requires the imaging system to include additional components and, thus, increases the cost and complexity of the system.
Third, known image contrast grids, such as the image contrast grid 124, only remove the Compton-scattered, non-normal (off-z-axis) photons in one dimension (i.e., along either the x-axis or the y-axis). In order to provide two-dimensional photon removal using these grids, two grids, such as two of the image contrast grids 124, have been stacked with their respective foils oriented orthogonal with respect to those of the other grid. Although the combined use of two grids may improve Compton-scattered photon removal in a second direction, the cost of the imaging system is also significantly increased by the added cost of the second grid. Thus, the value of improving the performance of the imaging system by using two image contrast grids may not justify the associated added cost to achieve the improved performance.
This invention provides improved image contrast grids that can overcome the above-described problems of the known image contrast grids and the processes used to form the known image contrast grids.
This invention separately provides image contrast grids that have improved x-ray transmission efficiencies, i.e., rejection ratios, that thus reduce the required dosage of source radiation that is needed to obtain an image of an object.
This invention separately provides image contrast grids that have increased open aperture ratios.
This invention separately provides image contrast grids that can be used to form images with improved contrast.
This invention separately provides image contrast grids that have fine structures that reduce or eliminate the need to use a Bucky system during imaging.
This invention separately provides image contrast grids that remove Compton-scattered x-rays in two, co-planar dimensions, e.g., the x and y dimensions, and thus eliminate the need to use two image contrast grids simultaneously.
This invention separately provides methods of making the image contrast grids that are economical, controllable and reproducible.
This invention separately provides methods of using the image contrast grids in imaging systems for imaging objects.
Various exemplary embodiments of the image contrast grids according to this invention comprises a body forming a continuous matrix and openings. The body comprises one of a first material that is at least substantially transparent to x-rays and a second material in the openings that absorbs the x-rays without substantially scattering the x-rays. Another of the first material and the second material is disposed in the openings. The body includes a first surface where the x-rays enter the image contrast grid and a second surface opposite to the first surface where the x-rays exit the image contrast grid. The openings extend at least partially from the first surface to the second surface.
In some exemplary embodiments, the first surface of the body is machined to provide enhanced focus capabilities.
This invention also provides x-ray imaging systems to image objects that comprise an x-ray source that emits x-rays and an image contrast grid positioned such that x-rays emitted by the x-ray source pass through the object and impinge on the first surface of the image contrast grid. An image plane faces the second surface of the image contrast grid.
In various exemplary embodiments of the x-ray imaging systems according to this invention, the image contrast grid can be maintained stationary during imaging without forming grid lines on the formed image of the object. Thus, in various exemplary embodiments of the x-ray imaging system, it is not generally necessary to use a Bucky system during imaging. The image contrast grids according to this invention remove Compton scattered x-rays that pass through the object in two coplanar dimensions of the image contrast grid.
Exemplary embodiments of the methods of making image contrast grids comprise forming the body including the openings. The x-ray absorbing material can be formed in the openings or the x-ray absorbing material can be used to form the body. The openings and the x-ray absorbing material can be formed by various exemplary embodiments of the methods according this invention.